Journal of the Royal Society of New Zealand abstracts

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A Jurassic chert-limestone-spilite association near Eketahuna, North Island, New Zealand

P. R. Moore*

Bedded chert and associated bioclastic limestone and spilite near Eketahuna are locally in depositional contact with enclosing Jurassic greywacke of the Tararua Formation (Torlesse Terrane). The chert contains abundant, moderately well preserved radiolaria of Middle Jurassic (Bajocian) age, and a Late Jurassic age is indicated for associated limestone.

Immobile elements indicate the spilitic rocks are strongly alkaline in character, and very different from many other basaltic lavas in the Torlesse Terrane. They can probably be classified as oceanic alkali basalts, but their original tectonic setting is not evident from the limited geochemical data. Vesicularity of the lavas suggests they may have been erupted at water depths of less than 500 m.

A depositional model is proposed in which lava erupted on the outer shelf or upper slope provided a temporary barrier against terrigenous sedimentation, enabling radiolarian chert to be deposited on the seaward (protected) side, and relatively pure bioclastic limestones to form on top of the volcanic high. This model does not require the chert-limestone-spilite association to be tectonically emplaced into the terrigenous clastic sequence as has been inferred for similar associations elsewhere in New Zealand.

Keywords: Chert; limestone; spilite; volcanic sediments; Tararua Formation; Torlesse Terrane; Jurassic; stratigraphy; petrography; geochemistry; chemical analyses; vesicularity; radiolaria; macrofossils; tectonic setting.

(c) Journal of The Royal Society of New Zealand,

Volume 25, Number 2, June 1995, pp 99-114

PDF file of entire paper: medium quality (2145K); (scanned from paper original: notes about this process)

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